1. Field of the Invention
The present invention relates to an image forming apparatus having plural optical scanning devices.
2. Description of the Related Art
Conventionally, electrophotographic tandem color image forming apparatuses in which different color toner images are formed on plural image bearing members arranged side by side and the color toner images are overlaid on a recording material to form a color image (such as full color images) are known. In such electrophotographic tandem color image forming apparatuses, optical scanning devices irradiate the surfaces of plural photoreceptors serving as the image bearing members with light beams according to image data, thereby forming electrostatic latent images on the photoreceptors. The electrostatic latent images on the photoreceptors are then developed by respective developing devices using different color toners to form color toner images on the photoreceptors, followed by transferring the toner images onto a recording material optionally via an intermediate transfer medium such as a belt and a drum. The toner images thus overlaid on the recording material are then fixed thereon by a fixing device, resulting in formation of a copy.
In a tandem image forming apparatus using an intermediate transfer medium, a primary transfer process in which different color toner images formed on plural photoreceptors are sequentially transferred onto the intermediate transfer medium so as to be overlaid, and a secondary transfer process in which the overlaid toner images are transferred onto a recording material at the same time are performed.
In a tandem image forming apparatus using a feeding member (such as a belt), a transfer process in which different color toner images formed on plural photoreceptors are sequentially transferred onto a recording material fed by the feeding member so as to face the photoreceptors, resulting in formation of overlaid color toner images on the recording material is performed.
After the transfer process, the color toner images overlaid on the recording material are fixed thereto upon application of heat and pressure, and the recording material bearing the fixed color image is discharged from the main body of the image forming apparatus.
Recently, such color image forming apparatuses are required to have small size, low costs and long life. In order to prolong the life of a photoreceptor, a method in which when a monochrome image is formed on a photoreceptor, other photoreceptors are separated from an intermediate transfer medium is used. In addition, a method in which the diameter of photoreceptors is increased, or a method in which the diameter of a most used photoreceptor for forming black color images is increased is used.
Among such image forming apparatuses, there is an image forming apparatus in which the optical scanning positions of plural photoreceptors at which light spots are formed on the photoreceptors are not present on a line (as illustrated in FIG. 3) in order to prolong the life of the photoreceptors. When an optical scanning device having only one polygon scanner is used for scanning plural photoreceptors in such an image forming apparatus, the light paths are very complicated. Therefore, the case holding the optical elements of the optical scanning device has to have a complicated shape, and adjustment of the optical elements is also complicated. In addition, in order to separate the light paths, it is necessary to use a thick polygon scanner or a two-tier polygon mirror, and a scanning lens having a complex shape.
In attempting to solve the problem, it is considered to use plural (four) optical scanning devices for scanning the respective (four) photoreceptors, or two optical scanning devices each scanning two photoreceptors. Namely, in such image forming apparatuses, two or four optical scanning devices are used for scanning four photoreceptors with respective four light beams. Such image forming apparatuses have a relatively simple configuration compared to image forming apparatuses having only one scanning device for scanning four photoreceptors, and can easily scan photoreceptors even when the lengths of light paths to the photoreceptors are different. Therefore, such image forming apparatuses have small size and low costs.
Recently, in order to control image quality, a technique in that adjustment of the light beam position is performed by changing the position or angle of optical elements of an optical scanning device is often used. Therefore, even when plural same optical scanning devices are used for an image forming apparatus, it is often necessary to make specific adjustment for each of the optical scanning devices depending on the position of the optical scanning device. In addition, in a case where the scanning (irradiation) positions of plural photoreceptors are not on a line and thereby the distances between optical scanning devices and the photoreceptors are different, it is necessary to perform major adjustments on the optical scanning devices (particularly optical elements of the optical scanning devices) depending on the positions thereof, so that desired beam spots are formed on the photoreceptors. When such an image forming apparatus is assembled, a method in which after plural same optical scanning devices, which have been assembled in the same way and then subjected to the same preliminary adjustment, are set in the image forming apparatus, specific adjustment is performed on each of the optical scanning devices can be used. The method is simple and the resultant image forming apparatus has acceptable optical scanning property. Therefore, the method is useful.
However, in a case where the optical scanning devices are detached from the image forming apparatus for any reason, it is preferable to set again the optical scanning devices to the original positions because it is not necessary to perform a readjustment operation on the thus reset optical scanning devices.
In particular, when such a readjustment operation (particularly, a mechanical adjustment operation) is performed on the optical scanning devices at a time other than the manufacturing process within a limited period of time using limited tools, the readjustment operation has to be carefully performed. For example, in a case where a dust protective cover for an optical scanning device is detached to access an optical element located in an inner portion of the optical scanning device, it is necessary to prevent dust from entering the optical scanning device and to carefully replace the dust protective cover.
Conventional image forming apparatuses using plural optical scanning devices often cause the following problems. Specifically, when the optical scanning devices are detached from the image forming apparatus, followed by attachment to the image forming apparatus, the optical scanning devices are not attached to the original positions, and therefore it becomes necessary to detach and reset the optical scanning devices, resulting in increase in maintenance costs. In addition, dust enters into the optical scanning devices due to the readjustment, resulting in deterioration of the properties of the optical scanning devices.
For these reasons, the present inventors recognized that there is a need for an image forming apparatus which uses plural same optical scanning devices and which does not cause the above-mentioned problems when the optical scanning devices are detached from the image forming apparatus and then attached again thereto.